The invention relates to a method of fabricating a semiconductor device, and more particularly relates to a method of fabricating a semiconductor device capable of improving the nitrogen (N) profile and degradation characteristics of a tunnel oxide layer.
In semiconductor devices, a flash memory device is a non-volatile memory device capable of storing information in a memory cell and performing an electrical erasing operation at a high rate when the memory device is mounted to a circuit board and the power is shutoff. Studies have focused on developing structures having an increased level of integration of the memory device. A unit cell of the above flash memory device is formed by laminating sequentially a tunnel oxide layer, a floating gate, a dielectric layer and a control gate on an active area of a semiconductor substrate. Unlike a gate insulating layer in a conventional transistor, the tunnel oxide layer in the above unit cell structure acts as a data conduit, and so an excellent layer characteristic of the tunnel oxide layer is required.
In a NAND flash memory device, the program and erase operations are performed using Fowler-Nordheim (F-N) tunneling. Accordingly, after numerous program and erase cycles, the tunnel oxide layer degrades and the memory device no longer functions properly. Therefore, to form the tunnel oxide layer of the flash memory device, a pure oxide layer is formed through a radical oxidation process and nitrogen is then implanted into the layer through an annealing process using nitrous oxide (N2O) gas. Due to the above processes, a charge trap density at the interface between the semiconductor substrate and the tunnel oxide layer decreases. Further, stress-induced leakage current (SILC) characteristics and capacitance-voltage (C-V) characteristics are improved to enhance both cycling characteristics and charge retention characteristics.
However, there is a limit to the increase in concentration of nitrogen accumulated in the oxide layer through the nitrous oxide (N2O) gas annealing process. To solve the above problem, an annealing process utilizing nitrogen monoxide (NO) gas having an excellent reactivity in place of nitrous oxide (N2O) gas is performed to implant nitrogen into the oxide layer has been employed. When nitrous oxide (N2O) gas is utilized in the annealing process, however, the tunnel oxide layer is degraded due to a change in the nitrogen profile in the tunnel oxide layer and a substitution of process gas.